This invention relates to a cryogenic vessel and particularly, to a cryogenic vessel for maintaining an electrical apparatus such as a superconducting coil at a cryogenic temperature.
FIGS. 1 and 2 illustrate a conventional cryogenic vessel disclosed in Japanese Patent Laid-Open No. 56-116555. The conventional cryogenic vessel comprises a liquid helium tank 10 having a substantially flat top wall 12, a bottom wall 14 and side walls 16. The liquid helium tank 10 is made of stainless steel since it must be made of a mechanically strong, highly weldable material. The liquid helium tank 10 hermetically contains therein an electrical apparatus 18 such as a superconducting coil to be maintained at a cryogenic temperature. The electrical apparatus 18 is surrounded and cooled by a cryogen 20 such as liquid helium at a cryogenic temperature.
The cryogenic vessel also comprises a radiation shield 22 enclosing the liquid helium tank 10 and an outer housing 24 enclosing the radiation shield 22. The radiation shield 22 is for shielding the cryogen tank 10 against heat radiation from the outer housing 24. The radiation shield 22 and the cryogen tank 10 are supported by a plurality of thermally insulating supports 26 in a thermally insulating relationship.
The cryogenic vessel further comprises a cooling tube 28 for passing therethrough evaporated cryogen or the cryogen gas 30 in the helium tank 10 along the radiation shield 22 for evenly cooling the radiation shield 22. The cooling tube 28 is generally coiled or wound in a serpentine manner and is bonded to the radiation shield 22 in a good heat conducting relationship so that substantially the entire surface of the radiation shield 22 is substantially uniformly cooled. The cooling tube 28 has its single inlet 32 open at the center of the top wall 12 of the liquid helium tank 10 and has its outlet 34 passing through the top wall of the outer housing 24 to communicate with the exterior of the housing 24.
Since there is a temperature difference of about 300.degree. K. between the outer housing 24 and the liquid helium tank 10, a large amount of radiant heat proportional to the difference between the fourth power of the absolute temperatures as understood from the Steeven Boltsmann' law would intrude into the helium tank 10 from the outer housing 24 to evaporate a large amount of the liquid helium 20 if no radiation shield 22 were provided. The radiation shield 22 serves to prevent the direct intrusion of the radiant heat from the outer housing 24 into the helium tank 10, the lower the temperature of the radiation shield 22, the smaller the amount of heat that intrudes into the liquid helium tank 10. Therefore, the radiation shield 22 is provided on its surface with the cooling tube 28 so that the radiation shield 22 can be cooled by cold helium vapour.
Since the conventional cryogenic vessel is constructed as described above, when the liquid volume tank 10 contains a sufficient amount of liquid helium 20 as shown in FIG. 1, the upper portion of the helium tank 10 above the liquid helium level is sufficiently cooled by the evaporated helium 30 above the level of the liquid helium 20. Therefore, the radiant heat coming into the liquid helium 20 from the outer housing 24 is sufficiently small and the amount of evaporation of the liquid helium 20 is small.
However, when the level of the liquid helium within the tank 10 becomes low as illustrated in FIG. 2 due to evaporation, the helium gas 30 flows substantially along the paths shown by the arrows from the surface of the liquid helium 20 toward the inlet 32 of the cooling tube 28. Therefore, the heliuim gas 30 does not sufficiently contact the upper corners of the helium tank 10, and these portions of the tank 10 are not sufficiently cooled. This insufficient cooling of the corner portions of the helium tank 10 is aggravated by the fact that the tank 10 is made of stainless steel which has poor heat conductivity. Therefore, the temperature of the upper corners of the helium tank 10 rises due to the radiant heat from the outer housing 24, so that the radiant heat intruding into the helium tank 10 increases, thereby further increasing the amount of evaporation of the liquid helium 20.